It is known practice to supply an industrial user with frigories using a working gas circulating in closed circuit or even in open circuit and subjected to a cooling process which generally relies upon a cycle comprising a compression followed by expansions and/or passes through heat exchangers.
In this regard, it is known practice to cause the working gas, after compression, to circulate through a cold box which may notably comprise expansion turbines and/or a plurality of heat exchangers.
However, one of the difficulties associated with the design and operation of such cryogenic installations stems from the need to meet contradictory requirements dependent on whether the refrigeration method is in a transient cooling state or a steady state (or “normal operation”) of maintaining a very low temperature.
Specifically, in the steady state, namely when the cryogenic installation is used only to sustain the supply of frigories to the user in order to keep and stabilize said user at a predetermined low operating temperature (for example of the order of 80 K), it is necessary to use very high-performance exchangers, typically brazed (wavy) plate and fin aluminum exchangers (“brazed aluminum heat exchangers”) which limit the pressure drops and optimize thermal efficiency.
Such aluminum exchangers do, however, suffer from certain limitations, notably due to the fact that they are mechanically unable to withstand the stresses resulting from a steep thermal gradient between the fluids passing through them, particularly when said fluids circulate countercurrentwise.
Now, significant temperature gradients occur precisely during the transient state, and notably during cooling, namely when the user needs to be brought down from a relatively high starting temperature (typically above 150 K and generally greater than or equal to 300 K) to a relatively low operating temperature (for example of the order of 80 K).
Of course, brazed aluminum exchangers need to be protected during this transient state, which may sometimes extend over a lengthy period and, for example, be as much as several tens of days in the case of a cryogenic installation used to cool superconductor magnets.
Within known cryogenic installations, it has therefore been envisaged, in order to reconcile the aforementioned requirements, for the equipment items to be duplicated and notably for one or more auxiliary cooling systems using volumes (baths) of liquid nitrogen to be added to the inlet of the cold box and for a complex switchover circuit to be provided that allows the stream of working gas to be directed selectively through said auxiliary systems, for the purpose of modifying the configuration of the cryogenic installation on a case-by-case basis according to the operating regime.
Despite such precautions, known cryogenic installations may exhibit uneven performance between the transient state and the steady state, being less well suited to one operating regime than to the other.
Furthermore, said cryogenic installations are very bulky and complex in structure and are expensive to install and to maintain.